Chemistry Reference
In-Depth Information
observed also with milk and whey powders (Berlin et al., 1968). In order to
obtain isotherms for truly amorphous lactose, particularly for the temperature
and RH range relevant to spray-drying conditions, dynamic methods were
proposed, allowing samples to reach equilibrium with the air in a short time [3 h
with a fluidized bed in sorption conditions (Kockel et al., 2002) or 50 min with a
thin film, in desorption (Lin et al., 2005)]. At low RH, amounts of water sorbed
by crystalline
-lactose monohydrate are only 100 times lower than by amor-
phous lactose, but strongly increase for RH
>
80%. The water is then retained
as capillary water between lactose particles, the water content being dependent
on particle size (Bronlund and Paterson, 2004).
An exhaustive review of studies on water sorbed on caseins and whey
proteins can be found in Kinsella and Fox (1986). Water sorption by caseins or
whey proteins yields sigmoidal SI, as for biopolymers in general (Figure 11.4).
The shape of the curves and the sorption rates vary with the composition of
powders and the method of preparation, although denaturation of the proteins
40
whey protein isolate
micellar casein
Na caseinate
35
30
25
20
15
10
5
0
0
10
20
30
40
50
60
70
80
90
Relative humidity (%)
Figure 11.4.
Water sorption isotherms for micellar casein and whey protein isolate (sorption
4-378C, Foster et al., 2005) and Na-caseinate (sorption 258C, Weisser, 1985). The lines are from
the GAB model.
